Subneptin is a type of exoplanet characterized by high birth discharge thrombosis and lacks analogs within our solar system. Significantly smaller than gas giants, and typically cooler than Hot Jupiter exoplanets, these worlds were notably challenging to study before the launch of the NASA/ESA/CSA James Webb Space Telescope. Many subneptins are obscured by thick clouds and hazards, hindering our ability to analyze their atmospheric structures. Utilizing the Webb, astronomers have obtained the transmission spectrum of subneptin TOI-421B, unveiling its atmospheric chemical signatures.
Artist’s impression of Subneptune Exoplanet TOI-421B. Image credit: NASA/ESA/CSA/DANI player, STSCI.
TOI-421 is a solar-type star located approximately 245 light years away in the constellation of Repas.
Commonly known as BD-14 1137, this star is around 10 billion years old and hosts at least two giant exoplanets.
The inner planet, TOI-421B, is a subneptin with a radius of 2.65 times that of Earth and boasts a high equilibrium temperature of 647 degrees Celsius (1,197 degrees Fahrenheit).
“Prior to Webb, scientists had scant information regarding subneptins,” stated University of Maryland astronomer Brian Davenport and his team.
“These planets are several times larger than Earth, yet still much smaller than gas giants, usually cooler than hot Jupiters, and significantly harder to observe than their larger gas analogs.”
“A crucial finding before Webb was that many Neptune-like atmospheres exhibited flat or featureless transmission spectra.”
“This indicates that when scientists scrutinized the spectrum of planets transiting in front of a host star, they only observed flatline spectra, missing the details of the spectrum (chemical fingerprints revealing atmospheric composition).”
“Based on these flatline spectra observations, it was concluded that certain subneptins are extremely obscured, potentially due to clouds or haze.”
“Why did we focus on planet TOI-421B? Because we hypothesized it might be an exception,” said Eliza Kempton, an astronomer at the University of Maryland.
“This hypothesis stemmed from previous data suggesting that planets within specific temperature ranges were less likely to be shrouded in haze or clouds.”
“The temperature threshold is around 577 degrees (1,070 degrees Fahrenheit); beneath this, it was assumed that complex photochemical reactions occur between sunlight and methane gas, leading to haze.”
TOI-421B, with a temperature of approximately 727 degrees Celsius (1,340 degrees Fahrenheit), is significantly above this threshold.
The transmission spectra of subneptune TOI-421B uncover the presence of water and potential indications of sulfur dioxide and carbon monoxide, without signs of carbon dioxide or methane. Image credits: NASA/ESA/CSA/Joseph Olmsted, STSCI.
Without the interference of haze or clouds, astronomers anticipated observing a clear atmosphere.
“We identified spectral features attributable to various gases, which empowered us to ascertain the atmospheric composition,” explained Davenport.
“In many previously studied subneptins, although I inferred that their atmospheres contained specific gases, they remained obscured by haze.”
Researchers have identified atmospheric water vapor along with tentative signatures of carbon monoxide and sulfur dioxide.
However, they did not find molecules such as methane and carbon dioxide.
From the gathered data, they speculate a substantial amount of hydrogen constitutes the atmosphere.
This prevalence of lightweight hydrogen was an unexpected revelation for scientists.
“We recently came to grips with the notion that one of the initial subneptins observed by Webb has a significant molecular atmosphere.
“This implies that TOI-421B may have formed and evolved differently compared to other cooler subneptins.”
“The hydrogen-rich atmosphere is intriguing, as it resembles the composition of its host star TOI-421B.”
“By incorporating the same gases that formed the host star into the planet’s atmosphere, and cooling them, one could replicate the same gas combination.”
“This process aligns more closely with the giant planets of our solar system, differing from previously observed subneptins through Webb.”
The team’s research paper was published this week in the Astrophysical Journal Letters.
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Brian Davenport et al. 2025. TOI-421B: High-temperature Neptune with a low average molecular weight atmosphere, haze-free. apjl 984, L44; doi: 10.3847/2041-8213/ADCD76
Source: www.sci.news
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